CRYSTAL GROWTH & DESIGN
Epitaxial ZnMnO/ZnO Coaxial Nanocable Duofa Wang,†,‡ Sangyoon Park,† Yongsu Lee,† Taiwoon Eom,† Seongjae Lee,† YoungPak Lee,*,† Cheljong Choi,§ Jinchai Li,| and Chang Liu| q-Psi and Department of Physics, Hanyang UniVersity, Seoul 133-791, Korea, Faculty of Materials Science and Engineering, Hubei UniVersity, Wuhan 430062, China, Department of Semiconductor Science and Technology, Chonbuk National UniVersity, Jeonju, 561-765, Korea, and Department of Physics and Key Laboratory of Acoustic and Photonic Materials and DeVices of Ministry of Education, Wuhan UniVersity, Wuhan 430072, China
2009 VOL. 9, NO. 5 2124–2127
ReceiVed June 26, 2008; ReVised Manuscript ReceiVed December 30, 2008
ABSTRACT: Zn0.96Mn0.04O/ZnO coaxial nanocables were prepared in two steps: ZnO nanowires were synthesized by standard thermal evaporation, followed by depositing Zn0.96Mn0.04 onto the surface of prepared ZnO nanowires using an ultrahigh-vacuum radio frequency magnetron-sputtering system. X-ray diffraction and X-ray photoelectron spectroscopy analysis reveal that Mn is incorporated well into the wurtzite ZnO without forming any Mn oxide. High resolution transmission electron microscopy images demonstrate that both ZnO and Zn0.96Mn0.04 layers are single-crystalline, and epitaxial growth is achieved between them, which predicts high carrier mobility and spin injection efficiency. Magnetic property measurements show that the Zn0.96Mn0.04/ZnO nanocables are ferromagnetic with a Curie temperature higher than 350 K.
1. Introduction Dilute magnetic semiconductors (DMSs) have attracted a great deal of attention in the past two decades as enabling materials in the emerging field of spintronics.1,2 DMSs are semiconductor solid solutions, where a small percentage of cations are replaced by magnetic impurities. Mn-doped ZnO, one of the DMSs, has attracted much interest because of the theoretical prediction of ferromagnetism above 300 K.3 However, a majority of this work focuses on the bulk or the thin film.4-6 To realize the advantages offered by spin, spintronic devices in the future require nanoscaled DMS materials, and the assembly and the electronic interconnection of these nanomaterials. Recently, the synthesis of ferromagnetic ZnMnO nanowires has been reported by several groups,7-9 but the assembly of DMS nanomaterials has not been attempted so far. Crossed,10,11 axial,12,13 and radial structures14,15 have been proposed to assemble nanomaterials. Among them, the radial structure, namely, nanocable is the most competitive, since it can provide a more efficient injection current with respect to the crossed and the axial structures.5 Up to now, CdS/Si,16 ZnO/ CdS,17 SiC/SiO,18 and Ni/ZnO19 coaxial nanocables have been synthesized by various methods. In the present work, Zn1-xMnxO/ZnO (x ) 0.04, 0.16) coaxial nanocables were prepared in two steps: ZnO nanowires were synthesized by a standard thermal evaporation technique, followed by depositing Zn1-xMnxO epitaxially onto the surface of the as-prepared ZnO nanowires.
2. Experimental Section The Zn1-xMnxO/ZnO coaxial nanocables were fabricated by sputtering Zn1-xMnxO onto the surface of ZnO nanowires in an ultrahigh-vacuum (UHV) radio frequency sputtering system. Before sputtering, the ZnO nanowires were made by thermal evaporation. * To whom correspondence should be addressed. Phone: +82-2-2281-5572. Fax: +82-2-2281-5573. E-mail:
[email protected]. † q-Psi and Department of Physics. ‡ Faculty of Materials Science and Engineering. § Department of Semiconductor Science and Technology. | Department of Physics and Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education.
A p-type silicon substrate was placed on a quartz boat loaded with Zn powders (purity: 99.99%; distance between the Zn source and the Si substrate: 2-3 mm), and then the quartz boat was transferred into the center of a tube furnace. Afterward, the chamber was pumped down and then heated to 500 °C at a rate of 25 °C min-1. Once the temperature reached 500 °C, O2 and Ar were introduced into the chamber as reactant and carrier gases, respectively. After the reaction continued for 1 h, the as-prepared ZnO nanowires were taken from the quartz tube and loaded into the cosputtering system as the final substrate. ZnO and Mn targets were used for the cosputtering. The shell layer of Zn1-xMnxO was prepared in the UHV system at a base pressure of 5.0 × 10-9 Torr. For the analysis of composition using Rutherford backscattering spectroscopy (RBS), a p-type Si (100) wafer was also mounted together with the asprepared ZnO nanowires to deposit ZnMnO thin film. During the sputtering process, the pressure in the chamber was kept at 4.0 × 10-4 Torr and the temperature of substrate was 300 °C. The content of Mn was controlled by changing the applied power for the Mn target. The morphologies of the core-shell nanocable and the bare ZnO nanowire were confirmed by scanning electron microscopy (SEM). The crystalline structure was investigated by X-ray diffraction (XRD), and the chemical state of Mn in the sample was studied by X-ray photoelectron spectroscopy (XPS). The microstructure of nanocable was obtained by high-resolution transmission electron microscopy (HRTEM). The composition analysis was carried out by RBS quantitatively and by energy-dispersive X-ray spectroscopy (EDX) elemental mapping qualitatively. The magnetic properties were measured with a superconducting quantum interference device magnetometer (Quantum Design, MPMS XL-7).
3. Results and Discussion Figure 1 illustrates XRD analysis of ZnO nanowire, and Zn0.96Mn0.04O/ZnO and Zn0.84Mn0.16O/ZnO coaxial nanocables. Only diffraction peaks corresponding to the wurtzite ZnO are observed in the XRD patterns and no any other phases relevant to Mn or Mn oxides are found within the detection resolution of XRD. Moreover, the (002) diffraction peak of Zn0.96Mn0.04O/ ZnO reveals a stronger intensity and a narrower full width at half-maximum compared to those of Zn0.84Mn0.16O/ZnO. It is suggested that Zn0.96Mn0.04O/ZnO has a better crystallinity. RBS was used to analyze the composition of Zn1-xMnxO/ZnO nanocable samples, since it is an advanced composition measurement method for low doping level. The contents of Mn are determined to be 4 and 16 at. %, according to the RBS
10.1021/cg800677q CCC: $40.75 2009 American Chemical Society Published on Web 03/24/2009
Epitaxial ZnMnO/ZnO Coaxial Nanocable
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Figure 1. XRD patterns of (a) ZnO nanowire and (b) Zn0.96Mn0.04O/ ZnO and (c) Zn0.84Mn0.16O/ZnO nanocable.
Figure 3. (a) TEM and (b) HRTEM images of Zn0.96Mn0.04O/ZnO nanocable. Inset in (a) is the Z-contrast image. Dashed line in (b) is the assumed interface between Zn0.96Mn0.04O and ZnO, based on the increase of the diameter. SEM images of (c) ZnO nanowire and (d) Zn0.96Mn0.04O/ZnO coaxial nanocable. Insets are the corresponding images in a high magnification.
Figure 2. XPS spectra of Mn 2p states for (a) Zn0.96Mn0.04O/ZnO and (b) Zn0.84Mn0.16O/ZnO nanocable samples.
measurement of ZnMnO films on Si deposited together with Zn1-xMnxO/ZnO nanocables. The chemical state of Mn in Zn1-xMnxO/ZnO nanocable sample was investigated by XPS, as shown in Figure 2. The binding energy of Mn 2p3/2 and Mn 2p1/2 states are located at 641.3 and 654.3 eV, respectively, indicating that the chemical state of Mn is Mn2+. Therefore, Mn is substituted at the Zn site or is in the form of MnO cluster. However, the former one is more possible, since no diffraction peak relevant to MnO was observed in the XRD pattern. TEM was employed to investigate the microstructure of Zn1-xMnxO/ZnO nanocable. No diffraction contrast between the ZnO core and Zn0.96Mn0.04O shell was observed in the TEM image of Zn0.96Mn0.04O/ZnO nanocable, even in the Z-contrast image (Figure 3a), due to the low doping level of Mn. On the other hand, from the SEM images of ZnO nanowire (Figure 3c) and Zn0.96Mn0.04O/ZnO nanocable (Figure 3d), it can be seen very clearly that Zn0.96Mn0.04O/ ZnO coaxial nanocables keep their morphology to that of the as-prepared uncoated ZnO nanowires, but the diameter of Zn0.96Mn0.04O/ZnO coaxial nanocable was increased uniformly along the axis by 40 nm. Therefore, it is suggested that the nanocable structures are formed indeed as expected and that the thickness of the Zn0.96Mn0.04O shell layer is about 20 nm. HRTEM images of Zn0.96Mn0.04O/ZnO coaxial nanocables, shown in Figure 3b, reveal that both ZnO core and ZnMnO shell are single-crystalline, and no precipitation of metallic MnO is found, confirming that Mn is substituted at
the Zn site. Moreover, an epitaxial growth is achieved between the core and shell without any dislocation. The remarkable epitaxy of Zn0.96Mn0.04O is ascribed to the elastic boundary condition of nanowire and the improved surface mobility at a temperature of 300°.15,20 These nanocables are expected to have high carrier mobility and spin injection efficiency, which are the key factors affecting the performance of the spintronic device, since the scattering relevant to defects is greatly decreased in these highly crystallized materials. ZnO nanostructures are attractive components for nanometer-scaled electronic and photonic device and have been used to fabricate a variety of nanodevices, such as field effect transistor, Schottky diode, and light-emitting device array.21-23 The prepared coaxial epitaxial nanocable should have great potential in its application to spintronic devices, for example, spin light emission diode and spin transistor. Figure 4a is the TEM image of Zn0.84Mn0.16O/ZnO nanocable, where the core-shell structure can be seen clearly. The dark and the bright parts correspond to ZnO core and Zn0.84Mn0.16O shell, respectively. The thickness of Zn0.84Mn0.16O is about 20 nm. The elemental mapping by EDX demonstrates that Mn is present uniformly in the nanocable. Quantitative analysis on the EDX data reveals that the content of Mn is 16 at. %, which is consistent with the RBS results. The HRTEM image of Zn0.84Mn0.16O/ZnO in Figure 4d shows that ZnO and Zn0.84Mn0.16O are single crystalline and that Zn0.84Mn0.16O is grown epitaxially on the surface of the ZnO core. Figure 5a shows the magnetic hysteresis (M-H) curves of Zn0.96Mn0.04O/ZnO coaxial nanocables, measured at 300 K. The diamagnetism signals from ZnO and Si were subtracted and the M-H signals are from the ZnMnO shell layers. The applied field was fixed parallel to the sample plane during the measurement. Ferromagnetic behaviors are clearly observed at 300 K. The coercive field is 98 Oe and the saturated magnetization (MS) is 25.1 emu/cc. Figure 5(b) is the temperature-dependent magnetization (M-T) curve of Zn0.96Mn0.04O/ZnO, which was obtained in a magnetic field of 100 Oe. The field-cooled (FC) and the zero-field-cooled (ZFC) magnetizations are separated until 350 K, indicating a Curie temperature (Tc) higher than 350 K. This is
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that no precipitation of metallic Mn or Mn oxides is found. The magnetic properties of Zn0.84Mn0.16O/ZnO sample was also studied to be nonferromagnetic. Since no donor or acceptor dopant was introduced intentionally in the nanocable, the carrier concentration and conductivity should be very low. Therefore, the ferromagnetic exchange interaction is properly explained by the bound magnetic polaron model.2 The exact mechanism for the different magnetizations of samples with different Mn contents is still under investigation. It is suggested that both different Mn contents and crystalline qualities might be the reason for that, since the magnetic properties of DMS is very sensitive to the Mn contents and the properties of carriers and defects in DMS.27-29
4. Summary Room-temperature ferromagnetic Zn0.96Mn0.04O/ZnO coaxial nanocables were synthesized by two steps. Both the Zn0.96Mn0.04O shell layer and ZnO core layer in the nanocable are single-crystalline and epitaxy growth is obtained between them. This kind of coaxial nanocable presents potential applications to spintronic devices based on a single nanostructure. Acknowledgment. The work was performed with financial support by MEST/KOSEF through Quantum Photonic Science Research Centre of Korea and by the National Natural Science Foundation of China (Grant No. 10575078).
Figure 4. (a) TEM image of Zn0.84Mn0.16O/ZnO nanocable. Elemental mapping of (b) Mn and (c) Zn of Zn0.84Mn0.16O/ZnO coaxial nanocable. (d) HRTEM image of Zn0.84Mn0.16O/ZnO.
Figure 5. (a) M-H and (b) M-T curves of Zn0.96Mn0.04O/ZnO nanocable.
comparable to recent reports on ZnMnO film.24,25 Metallic Mn and all possible Mn oxide phases are antiferromagnetic, except Mn3O4 which is ferromagnetic only below 45 K.26 Therefore, the high-temperature ferromagnetism in the prepared coaxial nanocables is due to the ferromagnetic interaction between the neighboring Mn dopants at the Zn sites rather than to the Mn-oxide impurities such as MnO and Mn3O4. The absence of transition from ferromagnetic to paramagnetic phase at 45 K conforms further the absence of Mn3O4. It is also supported by the HRTEM results in Figure 3b that the shell layer is single-crystalline Zn1-xMnxO and
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